Fiber-coupled scintillation detectors of radiation and particles have been employed over the course of the past 30 years. In some cases, the scintillator is pixelated, consisting of discrete scintillator elements, and in other cases, other stratagems are employed (such as orthogonally crossed coupling fibers) in order to provide spatial resolution. Examples of fiber-coupled scintillation detectors are provided by U.S. Pat. No. 6,078,052 (to DiFilippo) and U.S. Pat. No. 7,326,9933 (to Katagiri et al.), both of which are incorporated herein by reference. Detectors described both by DiFilippo and Katagiri et al. employ wavelength-shifting fibers (WSF) such that light reemitted by the core material of the fiber may be conducted, with low attenuation, to photo-detectors disposed at a convenient location, often distant from the scintillator itself. Spatial resolution is of particular value in applications such as neutron imaging. Spatial resolution is also paramount in the Fermi Large Area Space Telescope (formerly, GLAST) where a high-efficiency segmented scintillation detector employs WSF readout for detection of high-energy cosmic rays, as described in Moiseev, et al., High efficiency plastic scintillator detector with wavelength-shifting fiber readout for the GLAST Large Area Telescope, Nucl. Instr. Meth. Phys. Res. A, vol. 583, pp. 372-81 (2007), which is incorporated herein by reference.
Because of the contexts in which fiber-coupled scintillator detectors have been employed to date, all known fiber-coupled scintillator detectors have counted pulses produced by individual interactions of particles (photons or massive particles) with the scintillator, thereby allowing the energy deposited by the incident particle to be ascertained based on the cumulative flux of light reemitted by the scintillator.
The detection requirements of x-ray backscatter inspection systems, however, are entirely different from the requirements addressed by existing fiber-coupled scintillation detectors. Backscatter x-ray inspection systems have been used for more than 25 years to detect organic materials concealed inside baggage, cargo containers, in vehicles, and on personnel. Because organic materials in bulk preferentially scatter x-rays (by Compton scattering) rather than absorb them, these materials appear as brighter objects in backscatter images. Insofar as incident x-rays are scattered into all directions, sensitivity far overrides spatial resolution as a requirement, and in most scatter applications, detector spatial resolution is of no concern at all, since resolution is governed by the incident beam rather than by detection.
The specialized detection requirements of large area and high sensitivity posed by x-ray scatter systems are particularly vexing in the case of “conventional” scintillation detectors 100 of the type shown in a side cross-section in FIG. 1A and in a front cross-section in FIG. 1B. An example of such a detector is described in U.S. Pat. No. 5,302,817 (to Yokota), and is incorporated herein by reference. Typically, a light-tight box 102 is lined with scintillating screens 103 where incident x-ray radiation 101 is converted to scintillation light, typically in the UV, visible, or longer wavelength, portions of the electromagnetic (EM) spectrum. Large-photocathode-area photomultiplier tubes (PMTs) 105 are coupled to receive scintillation light via portholes 108. One problem lies in that a fraction of the scintillation light originating within the screen is transmitted from the screen into the enclosed volume. The remaining scintillation light is lost in the screen material. Scintillating screens 103 are designed to maximize the fraction of emitted light, which is tantamount to ensuring a large transmission coefficient T for the interface between screen 103 and the medium (typically air) filling the detector volume. However, in a conventional backscatter detector of the sort depicted in FIGS. 1A and 1B, the scintillation screens 103 should also serve as good reflectors because scintillation light, once emitted into the volume of box 102, typically needs multiple reflections until it reaches a photo-detector 105. So, the reflection coefficient R of the screen surface should also be large, however, since the sum of T and R is constrained to be unity, both T and R cannot be maximized simultaneously, and a compromise must be struck. As a result, the light collection efficiency of the conventional backscatter detector is inherently low, with only a few percent of the generated scintillation light collected into the photo detectors.
For an imaging detector, the photon statistical noise is calculated in terms of the photons absorbed by the detector and used to generate the image. Any photons which pass through the detector without being absorbed, or even those that are absorbed without generating image information, are wasted and do not contribute to reducing noise in the image. Since photons cannot be subdivided, they represent the fundamental quantum level of a system. It is common practice to calculate the statistical noise in terms of the smallest number of quanta used to represent the image anywhere along the imaging chain. The point along the imaging chain where the fewest number of quanta are used to represent the image is called a “quantum sink”. The noise level at the quantum sink determines the noise limit of the imaging system. Without increasing the number of information carriers (i.e., quanta) at the quantum sink, the system noise limit cannot be improved. Poor light collection can possibly create a secondary quantum sink, which is to say that it will limit the fraction of incident x-rays resulting in PMT current. Moreover, it will increase image noise. Light collection efficiency can be improved by increasing the sensitive area of the photo-detectors, however, that path to efficiency is costly.
The structure of scintillating screen typically employed in prior art x-ray scintillation detectors is now described with reference to FIG. 2. A layer of composite scintillator 202 is sandwiched between a backer sheet 204 for structural support and a thin, transparent protective film 206 composed of polyester, for example. The composite scintillator consists of typically micron-size inorganic crystals in an organic matrix or resin. The crystals are the actual scintillating material. Barium fluoro-chloride (BaFCl, or “BFC”) or gadolinium oxysulfide (Gd202S, or “Gadox”) doped with rare earth elements are common choices for these. The stopping power of the screen is determined by the thickness of the composite scintillator layer 202, which is typically measured in milligrams of scintillator crystal per unit area. Because the inorganic scintillators (such as BFC or Gadox) suffer from high self-absorption, the composite scintillator layer has to be kept rather thin in order to extract a good fraction of the scintillation light. This limits the useful stopping power of the screen and makes it suitable only for the detection of x-rays with energies up to around 100 keV.
Therefore, it would be advantageous to have a scintillation detector for x-ray scatter detection applications that provides for more efficient extraction, collection, and detection of scintillation light.
As briefly discussed at the outset above, wavelength-shifting fibers (WSF) have long been employed for scintillation detection. Wavelength shifting fibers consist of a core with relatively high refractive index, surrounded by one or more cladding layers of lower refractive index. The core contains wavelength-shifting material, also referred as dye. Scintillation light which enters the fiber is absorbed by the dye which, in turn, emits light with a longer wavelength. The longer wavelength light is emitted isotropically in the fiber material. Total internal reflection traps a fraction of that light and conducts it over long distances with relatively low loss. This is possible, as described with reference to FIG. 3, because the absorption 304 and emission 302 wavelength ranges of the dye effectively do not overlap so that the wavelength-shifted light is not reabsorbed. The captured fraction is determined by the ratio of the refractive indices at surfaces of the fiber. An additional advantage of WSF is that the wavelength shifting can bring the scintillation light 306 into the sensitive wavelength range of the photo detector (PMT, silicon photomultiplier (SiPM), or Mutiple-Pixel Photon-Counter (MPPC), or otherwise).
Scintillator structures have been produced using many manufacturing technologies, including, for example, die-casting, injection molding (as described by Yoshimura et al., Plastic scintillator produced by the injection-molding technique, Nucl. Instr. Meth. Phys. Res. A, vol. 406, pp. 435-41 (1998), and extrusion, (as described in U.S. Pat. No. 7,067,079, to Bross, et al.), both of which references are incorporated herein by reference.